Alveolar macrophages have a fundamental role in regulating the innate host response in host defense against infection. However, there are also circumstances where chronic inflammation and fibrosis persist in the absence of infection. We have found that the extracellular matrix glycosaminoglycan hyaluronan (HA) is a critical regulator of lung injury, inflammation and fibrosis. We have found that HA fragments mediate lung inflammation by interacting with Toll-like receptors 2 and 4 on macrophages to produce inflammatory chemokines in a manner that depends upon the intracellular adaptor protein MyD88. In addition, we have made the unexpected observation that high molecular mass HA on the cell surface of lung epithelial cells is protective against non-infectious lung injury in a TLR2/4/MyD88 manner that appears to involve the constitutive activation of NF-?B. These data suggest that the TLR2/4-MyD88 pathway is a critical regulator of non-infectious lung inflammation through distinct mechanisms on macrophages and epithelial cells. We have also made the observation that TLR4 is protective against lung fibrosis by a mechanism that may involve a MyD88-independent pathway. Collectively, these data have led us to the hypothesis that extracellular matrix has a fundamental role in regulating lung injury, inflammation and repair by distinct interactions with TLRs on infiltrating macrophages and parenchymal epithelial cells. We will test this hypothesis in the following Specific Aims: 1. Define the role of MyD88 signaling in lung epithelial cells and macrophages in regulating non- infectious lung injury and repair using gene targeted mice that only express MyD88 in either the lung epithelium or in tissue macrophages. 2. Characterize the mechanisms of exaggerated fibrogenesis in the absence of TLR4 signaling following non-infectious lung injury. 3. Define the mechanisms by which hyaluronan-TLR interactions maintain alveolar homeostasis in response to non-infectious lung injury. PUBLIC HEALTH RELEVANCE. Chronic lung inflammation contributes to significant morbidity in lung diseases such as pulmonary fibrosis, asthma and COPD. This grant proposal is to find to treatments for lung inflammation by understanding how injured lung tissue causes chronic lung inflammation.